1. Field of the invention
The present invention relates to a thermoplastic resin composition containing fibrous magnesium oxysulfate which is made to be resistant to coloring. In more detail, the present invention relates to a thermoplastic resin composition employable for the production of a thermoplastic resin molding article which is excellent in various properties such as rigidity, mechanical strength, heat resistance and resistance to coloring.
2. Description of prior art
It is well known to incorporate inorganic materials into thermoplastic resins for the purpose of improving various properties of the thermoplastic resins, such as rigidity, mechanical strength, heat resistance, molding shrinkage and dimensional stability. Examples of the inorganic materials employable for that purpose include particulate substances such as calcium carbonate, barium sulfate and magnesium hydroxide; plate-shaped or flake-shaped substances such as talc and mica; and fibrous substances such as glass fiber and asbestos.
The particulate substances, among these inorganic materials, hardly give high reinforcing effect to the thermoplastic resins, so that the thermoplastic resin molds obtained by adding the particulate substances to the resins cannot be always employed for industrial materials requiring high quality. On the other hand, the plate-shaped or flake-shaped substances and the fibrous substances show high reinforcing effect in one- or two-dimensional directions, so that these substances are widely used as reinforcements for thermoplastic resins of high quality. However, since the above-mentioned reinforcements showing high reinforcing effect also have various drawbacks, they are sometimes restricted in their use depending on the purpose. For example, products obtained by molding the thermoplastic resins containing the plate-shaped or flake-shaped substances, are apt to have flow marks occurring in the molding procedure and generally show low rigidity as compared with molded products obtained from the thermoplastic resins containing fibrous substances such as fiber glass. On the other hand, the thermoplastic resins containing fibrous substances are molded to show poor elongation and to likely have silver streaks on the surfaces of the molded products. Further, the surfaces of the molded products mostly exhibit low glossiness. Particularly in the case of using a glass fiber, the resulting molded product shows low resistance to falling ball impact. Accordingly, in the use of the above-mentioned conventional reinforcements, it is necessary to make an appropriate selection according to the purpose.
The above-mentioned various drawbacks can be removed to a certain extent by appropriately selecting or modifying base materials, subjecting inorganic materials to be used as the reinforcements to surface treatment, adding other appropriate additives, adjusting molding and processing conditions, etc., but satisfactory removal of the drawback can be hardly obtained.
As a thermoplastic resin composition which is improved in various drawbacks found in conventional thermoplastic resin compositions containing the above-mentioned reinforcements, a thermoplastic resin composition having a fibrous (needle-like) magnesium oxysulfate has been proposed.
A polypropylene composition using the fibrous (needle-like) magnesium oxysulfate and crystalline polypropylene as a thermoplastic resin, which is improved in the above various properties, for example, is disclosed in Japanese Patent Provisional Publication No. 62(1987)-9260, and a composition of polyvinyl chloride containing a fibrous (needle-like) magnesium oxysulfate which is enhanced in rigidity is disclosed in Japanese Patent Provisional Publication No. 59(1984)-96153.
However, in the case of kneading fibrous or needle-like magnesium oxysulfate into a melted thermoplastic resin, the above resin compositions are apt to be colored.
A method improved in the above problem is proposed in Japanese Patent Provisional Publication No. 3(1991)-21646 in which the method of treating (coating) a fibrous (needle-like) magnesium oxysulfate employed for a thermoplastic resin component with an anionic surface-active agent is described. However, the kneading of the magnesuim oxysulfate in a melted thermoplastic resin removes a portion of the coating from the coated magnesium oxysulfate. Therefore, the surface active agent cannot bring about a sufficiently excellent effect of preventing the composition from coloring.